Pressure sensitive copying papers sometimes also called carbonless copy papers are widely used. A pressure sensitive copying paper system typically consists of a transferring sheet and a receiving sheet. One side of the transferring sheet is coated with microcapsules containing usually unreacted oil soluble coloring matter of the electron donor type which is encased in a thin wall of a polymeric material or gelatin or the like. The receiving sheet on the other hand is coated on one side with a special mineral or clay or polymer coating of the electron acceptor type and a binder. The sheets are superimposed so as to obtain an intimate contact between the two coated surfaces. If pressure is applied to these sheets by impact computer printout or typewriting or handwriting the microcapsules are ruptured in the impact or pressure areas and the coloring matter is transferred from the ruptured capsules to the adjacent mineral or clay or polymer coating of the receiving sheet whereby the electron donor type coloring matter is caused to react with the electron acceptor coating of the receiving sheet and as a result of this a colored localised marking corresponding to the impact or pressure area is formed on the receiving sheet. A pressure sensitive copying paper system consists basically of a first or top sheet coated on its reverse side this being the transfer surface with the microcapsules and the second sheet coated on its front side this being the receiving sheet with the mineral or clay or polymer coating. Pressure sensitive copying papers are particularly adaptable to manifolding where the papers form a multi-ply set, such as for instance a computer printout set or a five part sales form, where three intermediate sheets are coated on their front sides with the clay coating and on their reverse sides with microcapsules whereas the top sheet is coated on its reverse side only with microcapsules and the bottom sheet coated only on its front side with the clay coating.
Methods of forming microcapsules and of encapsulating a variety of substances are described for instance in U.S. Pat. Nos. 2,183,053, 3,016,308, 3,429,827, 3,516,941 and 3,533,958.
The copies produced by the use of the above described pressure sensitive copying paper have certain shortcomings in that for instance it is not possible to produce high intensity images, recorded information is not permanent as it is generally subject to light fading, thermal fading, molecular disintegration and the like. In addition the image density is generally inadequate for such information to be processed through an optical character recognition device. Further the image composition is not suited to magnetic image character recognition.